As electrolyte solutions used for electrochemical devices such as electric double layer capacitors, aqueous electrolyte solutions and non-aqueous electrolyte solutions are known. Non-aqueous electrolyte solutions have more advantages in having a decomposition voltage of 2 to 3 V while aqueous electrolyte solutions have a decomposition voltage of about 1.2 V. In other words, non-aqueous electrolyte solutions have advantages in having a higher withstand voltage than aqueous electrolyte solutions and thus have been rapidly spreading.
Sulfone compounds are generally used as extract solvents and solvents for various reactions. Particularly, sulfone compounds having a high dielectric constant are also used as aprotic polar solvents for non-aqueous electrolyte solutions. Among sulfone compounds, sulfolane, which has a high decomposition voltage, has been widely used as aprotic polar solvents for non-aqueous electrolyte solutions that are used as electrolyte solutions for electrochemical devices and the like.
The following are disclosed examples of electrochemical devices each containing a sulfone compound (e.g. sulfolane) as an aprotic polar solvent for an electrolyte solution: an electric double layer capacitor and an aluminum electrolytic capacitor, both of which include a mixture containing sulfolane or a sulfolane derivative such as 3-methyl sulfolane (Patent Literature 1, Patent Literature 2); an electric double layer capacitor that contains a solvent mixture of sulfolane or a sulfolane derivative (e.g. 3-methyl sulfolane) and propylene carbonate or the like (Patent Literature 3); and an aluminum electrolytic capacitor that contains a solvent mixture of sulfolane and a benzyl alcohol or a derivative thereof (Patent Literature 4).